Graphite oxide is a material obtained by oxidation of natural or synthetic graphite. The different oxidation procedures are named according to their respective inventors, i.e. the Brodie method, the Staudenmaier method, the Hummer and Offeman method [1]. Graphite oxide consists of multiple stacked graphene sheets bearing oxygen containing functional groups such as epoxy, hydroxyl or carboxylic moieties. Individual sheets of graphite oxide will be referred to as graphene oxide in the following.
Graphite oxide is a long known material that regained significant attention since Andre Geim and Konstantin Novoselov were awarded the Nobel price of physics 2010 for the discovery of graphene, a one-atom-thick planar sheet of sp2-bounded carbon [2]. Since then many fields ranging from optical to electrochemical applications have benefited from the implementation of graphene, owing to the outstanding transport and mechanical properties of the material [3].
Graphite oxide can be conveniently used as graphene precursor, especially for the production of bulk quantities. The exfoliation of graphite oxide to graphene oxide can be carried out in aqueous medium in concentrations up to 1 g/L [4]. The thermal or chemical reduction of graphene oxide yields reduced graphene oxide (RedGO) [5]. The C:O ratio rises from about 3 in graphite oxide to higher than 8 if graphite oxide is (i) heated to more than 1000° C. or (ii) treated hydrothermally at 150° C. or (iii) exfoliated to graphene oxide and reduced chemically [6]. While graphene oxide is an insulator, the electronic conductivity of RedGO is sufficient for most applications, such as use in electrochemical storage devices [4].
However, the composite formation of graphene oxide with a selected material and the subsequent reduction of graphene oxide to RedGO are by no means trivial. In fact, the prior art teaches reduction of graphene oxide to RedGO before or at least in an early step of the composite formation [7][8][9][10][11]. Additionally, a heating step to at least 300° C. is commonly applied to maximize the electronic conductivity of the composite [12].
The heating step to at least 300° C. restricts the type of material to be used in a composite, to materials that can withstand 300° C. and more. Above all, the main drawbacks in the composite formation procedures of the prior art originate from the fact that RedGO is used instead of graphene oxide. RedGO, like graphene sheets, graphene nano platelets and the like cannot be dispersed in high concentration and in a monolayer form in water without using surfactants, organic solvents or other chemicals generally acting as stabilizers to prevent aggregation and/or precipitation [7][8][9][10].
Due to these surfactants, organic solvents and/or other chemicals, the prior art fails to keep the carbon content of RedGO composites below 5%, which is highly desirable for their implementation in electrochemical storage devices. Further, the presence of reducing agents and/or stabilizers in the reaction mixture used for composite formation implies a work up and/or purification steps such as pyrolysis of the surfactants. Thus the high RedGO content and the complexity of the procedures in the prior art counterbalance the advantage of using graphene or reduced graphene oxide, which are expensive materials with unique properties.
Therefore there is still a need for an improved composite material comprising at least one Electrochemically Active Material (EAM) in nanoparticulate form and RedGO, especially for an improved composite material comprising at least one EAM in an anisotropic form like fibers and RedGO, as well as a method for its manufacture.